The present invention relates generally to shielded ribbon cables, and more particularly to shielded ribbon cables which display improved consistency of electrical properties such as skew.
The requirement for a large capacity of signal distribution in a compact cable has been met with the use of a "ribbon" cable in which a large number of conductors (often 50 or more) lie in a single plane and are encased in a common insulating material. The ribbon cable provides many signal conductors in a compact cable while affording ease of terminability with mass termination equipment.
Electrical cables, especially those cables used for high speed data transmission, are extremely sensitive to changes in their electromagnetic environment. Changes or discontinuities in the electromagnetic environment of a cable may cause electrical properties of the cable to vary, such as the speed with which an electrical signal propagates through a conductor. In multiple conductor ribbon cables, such variation of electrical properties causes undesired results, including skewing of electrical signals. In addition to skewing in cables having a relatively large number of conductors, it should be noted that discontinuities in the electromagnetic environment of a cable having only a few conductors are also problematic, and the invention described herein is equally applicable to cables having only a few conductors.
Skew is defined as the difference in arrival time of pulses at the far end of a cable when the pulses are simultaneously launched from the near end of the cable. Typical unshielded flat ribbon cables will have skew ranging from about 10 to 20% of the total propagation time. Shielded cables, which utilize metallic, i.e., highly conductive, materials to shield the conductors will typically have skew of about 5 to 10%. The reduced skew of shielded cables is a result of the cable shielding stabilizing the electromagnetic environment of the cable conductors. The shielding allows the conductors in the cable to experience a uniform electromagnetic environment, and thus to exhibit uniform electrical properties.
The property of skew is important in digital signal transmission. For example, if signal pulses are simultaneously launched along parallel conductors at the near end of a ribbon cable, system designers want all of the pulses to arrive at the far end of the cable within a maximum time of about 25% of the total pulse length. To achieve this requirement, system designers are currently forced to restrict the maximum cable length or to use cables with more transmission speed consistency. As signal speeds increase (i.e., the signal pulses become shorter in duration and closer together in time), the consistency of the cable transmission speed becomes very important.
In the past, when interconnections which had low skew were required, it was common to cut individual signal paths to a specific electric length. Consider, for example, a system requiring a 40 foot path and using a pulse length of 1600 picoseconds. The maximum acceptable variation of pulse arrival time is 400 picoseconds (25% of the 1600 picosecond pulse length). If the propagation velocity of the pulse in the cable is 1500 picoseconds/foot, the control accuracy required is: ##EQU1##
If multiple conductors are to be cut to length with this accuracy, the length of each conductor must be cut within 3.2 inches (0.67% of 40 feet) to meet the control accuracy requirements, assuming there is no electrical variation in the conductors.
In this example, ribbon cables have a distinct advantage over single conductor cables because the multiple conductors in a ribbon cable can easily be cut to length with a variation of less than 0.5 inches. In contrast, individual conductors that are 40 feet long may stretch several inches depending upon how much tension the conductors are placed under during measuring and cutting, thereby increasing the difficulty of cutting the conductors to a consistent length.
Although ribbon cables greatly increase the ease of cutting multiple conductors to a uniform length, the multiple conductors of the ribbon cable must also exhibit electrical uniformity to provide the desired control accuracy. To achieve satisfactory electrical uniformity in a cable, several relevant physical properties of the cable must be very carefully controlled. Factors that are important for controlling electrical uniformity are the dielectric constant of the cable insulation, the distance between the electromagnetic shielding and the individual conductors, and the relative quantities of the various dielectric materials in the cable.
U.S. Pat. Nos. 4,475,006 and 4,533,784, each assigned to Minnesota Mining and Manufacturing, the assignee of the present application, disclose shielded ribbon cables and means for controlling electrical properties in ribbon cables. U.S. Pat. No. 4,475,006 discloses a flexible ribbon cable having a plurality of conductors lying in a single plane and surrounded by insulation. The outer surface of the insulation is flat on both sides of the cable, and there is a sheet conductor (typically a metal foil) adhesively bonded to the surface of the insulation about the entire circumference of the cable. The sheet conductor is cigarette-wrapped along the length of the cable with an overlap at the seam of the cigarette wrap. Alternatively, separate layers of the sheet conductor are placed on each major surface of the cable, with the two shield layers overlapping and contacting at the edges of the cable.
U.S. Pat. No. 4,533,784, discloses a shielded ribbon cable having an extensible electromagnetic shield. Sometimes referred to as a pleated foil cable (PFC), the cable shield has multiple pleats which are transverse to the conductors, such that the shield has improved flexibility to reliably maintain integrity of the shield. Like the cable in U.S. Pat. No. 4,475,006, the shield is cigarette-wrapped about the cable insulation.
The cable construction of U.S. Pat. Nos. 4,475,006 and 4,533,784 provides works well and provides substantial improvement over conventional cables. Typically, cables made as described in of U.S. Pat. Nos. 4,475,006 and 4,533,784 have skew control of about 2.5% consistently, as compared to other shielded cables which provide skew control of 5 to 10%. However, in the example given above, a skew control of 0.67% was required. Thus, the skew control of cables having the construction described in U.S. Pat. Nos. 4,475,006 and 4,533,784 is inadequate for many modern system designs. To meet the requirements set out in the example, the skew control must be reduced by a factor of four (to provide skew of less than 0.67%). Or else the maximum cable length must be restricted to 10 feet (1/4 of 40 feet).
Prior art ribbon cables have not provided adequate skew control for modern electrical systems. What is needed is a ribbon cable which provides improved electrical consistency, and in particular a ribbon cable which provides improved skew control to meet the more demanding requirements of digital signal transmission.